1. Field of the Invention
This invention relates to an outboard motor steering system.
2. Description of the Related Art
Almost all outboard motor steering systems have up to now been of types operated by human power, such as the tiller handle type used to turn the rudder by manually operating a tiller handle attached to the outboard motor and the remote control type used to remotely operate a steering mechanism through a push-pull cable. Further, an add-on mechanism constituted as a separate unit from the outboard motor and used to power-assist the turning of the tiller handle is known. As shown in FIG. 23, this mechanism typically includes a steering hydraulic cylinder 204 attached to the front (boat side) of an outboard motor 200 through stays 202, and a link mechanism 206 connected between the hydraulic cylinder 204 and a tiller handle 208.
Human-powered steering systems are disadvantageous because they tend to have an unpleasant steering “feel” owing to, for instance, heavy steering load. The add-on steering system using a hydraulic cylinder also has disadvantages, most notably that its structure is complicated, that it adds to the number and weight of the components, and that it takes up space between the front of the outboard motor and the rear of the boat.
Attempts have been made to overcome these drawbacks. Japanese Laid-Open Patent Application No. Hei 2(1990)-279495 ('495), for example, teaches a steering system including a steering hydraulic cylinder attached to a swivel bracket. A piston rod of the hydraulic cylinder is extended and contracted to power-assist rudder turning through a tiller handle or the like. Since the steering system is integrated with the outboard motor, it minimizes increase in the number and weight of the constituent components and saves space.
In a hydraulic cylinder used for rudder turning, extension/contraction of the piston rod is generally controlled by on/off operating a control valve provided in an oil line of the hydraulic circuit and switching the rotation direction of the motor that drives the hydraulic pump.
There is also known a power tilt-trim system for regulating an outboard motor's tilt angle and trim angle. As set out in Japanese Laid-open Patent Application No. Hei 7(1995)-228296, for instance, this power tilt-trim system achieves light weight and compact size by utilizing the same hydraulic pump to supply hydraulic oil to both the hydraulic cylinder for regulating the tilt angle and the hydraulic cylinder for regulating the trim angle.
However, the steering system taught by '495 is disadvantageous from the aspect of saving space around the outboard motor because in some operating states of the hydraulic cylinder, the hydraulic cylinder projects from the outboard motor main unit. In addition, when two outboard motors are installed side by side in a dual motor configuration, the installation space must be enlarged by the amount of projection of the hydraulic cylinders so as to prevent interference between the outboard motors.